It's in the genes, stupid!

HUMAN INTELLIGENCE Kevin Mitchell, a developmental neurogeneticist, proposes that instead of thinking about the genetics of intelligence, we should be trying to decode the genetics of stupidity. The premise for this argument is that once natural selection generated the genes that build our big, smart human brains, those genes became “fixed” in the human population; observes David Dobbs

Few of us are as smart as we’d like to be. You’re sharper than Jim (maybe) but dull next to Jane. Human intelligence varies. And this matters, because smarter people generally earn more money, enjoy better health, raise smarter children, feel happier and, just to rub it in, live longer as well. But where does intelligence come from? How is it built?

Researchers have tried hard to find the answer in our genes. With the rise of inexpensive genome sequencing, they’ve analysed the genomes of thousands of people, looking for gene variants that affect intelligence, and have found a grand total of two. One determines the risk of Alzheimer’s and affects IQ only late in life; the other seems to build a bigger brain, but on average it raises IQ by 1.29 points.

Other genetic factors may be at work: A report last year concluded that several hundred gene variants taken together seemed to account for 40 to 50 per cent of the differences in intelligence among the 3,500 subjects in the study.

But the authors couldn’t tell which of these genes created any significant effect. When they tried to use the genes to predict differences in intelligence, they could account for only one per cent of the differences in IQ.“If it’s this hard to find an effect of just one per cent,” Robert Plomin, a professor of behavioural genetics at King’s College London, told New Scientist, “what you’re really showing is that the cup is 99 per cent empty.” But is the genetic cup really empty, or are we just looking for the wrong stuff? Kevin Mitchell, a developmental neurogeneticist at Trinity College Dublin, thinks the latter.

In an essay on his blog, ‘Wiring the Brain’, Mitchell proposed that instead of thinking about the genetics of intelligence, we should be trying to parse “the genetics of stupidity.” We should look not for genetic dynamics that build intelligence but for those that erode it.

The premise for this argument is that once natural selection generated the set of genes that build our big, smart human brains, those genes became “fixed” in the human population; virtually everyone receives the same set, and precious few variants affect intelligence. This could account for the researchers’ failure to find many variants of measurable effect.

Mutations: Do they matter?

But in some other genetic realms we do differ widely, for example, mutational load  the number of mutations we carry.

This tends to run in families, which means some of us generate and retain more mutations than others do.

Among our 23,000 genes, you may carry 500 mutations while I carry 1,000. Most mutations have no effect. But those that do are more likely to bring harm than good, Mitchell said, because “there are simply many more ways of screwing something up than of improving it.”

Smart Jane may be less a custom-built, high-performance model than a standard version pulling a smaller mutational load. We also inherit, through genes yet to be identified, a trait known as developmental stability. This is essentially the accuracy with which the genetic blueprint is built.

Developmental stability keeps the project on track. It reveals itself most obviously in physical symmetry. The two sides of our bodies and brains are constructed separately but from the same 23,000-gene blueprint. If you have high developmental stability, you’ll turn out symmetrical. Your feet will be the same shoe size, and the two sides of your face will be identical.

If you’re less developmentally stable, you’ll have feet up to a half-size different and a face that’s like two faces fused together. Doubt me? Take a digital image of your face and split it down the middle. Then make a mirror-image copy of each half and attach it to its original. In the two faces you’ve just made – one your mirrored left side, the other your right – you’ll behold your own developmental stability, or lack thereof.

Both those faces might be better-looking than you are, for we generally find symmetrical faces more attractive. It also happens that symmetry and intelligence tend to run together, because both run with developmental stability.

We may find symmetrical faces attractive because they imply the steadiness of genetic development, which creates valuable assets for choosing a mate, like better general fitness and, of course, intelligence  or as Mitchell might put it, a relative lack of stupidity

These ideas don’t strike geneticists as radical or contrary. Leonid Kruglyak, a Princeton geneticist, noted in an email that geneticists had long recognised that mutations could “throw sand in the gears of the brain” and that complex traits arose in complicated ways.Not brand-new, perhaps, but it’s this kind of “inversion of thought” (as Janet Kwasniak, a retired biologist, put it on her neuroscience blog, ‘Thoughts on Thoughts’) that can often spark new approaches to intractable problems.